Novel open-celled microporous film

ABSTRACT

Open-celled microporous polymer films characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above about 30 percent, a pore size of less than 5000 Angstroms and a nitrogen flux of greater than 35.4, are prepared by the consecutive steps of cold stretching, hot stretching and heat setting a non-porous, crystalline, elastic film.

United States Patent [1 1 Druin et al.

[ Apr. 2, 1974 NOVEL OPEN-CELLED MICROPOROUS FILM [75] Inventors: MelvinL. Druin, West Orange; John T. Loft, Springfield; Steven G. Plovan,Livingston, all of NJ,

[73] Assignee: Celanese Corporation, New York,

[22] Filed: July 13, 1972 [21] Appl. No.: 271,476

Related U.S. Application Data [60] Division of Ser. No. 84,712, Oct. 28,1970, Pat. No. 3,679,538, which is a continuation-in-part of Ser. No.876,511, Dec. 13, 1969, abandoned.

[52] U.S. Cl 156/229, 156/196, 156/212, 156/244, 161/402 [51] Int. Cl.B32g 3/02 [58] Field of Search 156/196, 212, 244, 229;

[56] References Cited UNITED STATES PATENTS 3,600,250 8/1971 Evans156/229 3,608,024 9/1971 Yazawa et a1. 156/229 Primary ExaminerEdward G.Whitby Attorney, Agent, or Firm-Thomas J. Morgan; Linn l. Grim; MarvinBressler [5 7 ABSTRACT Open-celled microporous polymer filmscharacterized by having a reduced bulk density as compared to the bulkdensity of the corresponding polymer films having no open-celledstructure, a crystallinity of above about 30 percent, a pore size ofless than 5000 Angstroms and a nitrogen flux of greater than 35.4, areprepared by the consecutive steps of cold stretching, hot stretching andheat setting a non-porous, crystalline, elastic film.

6 Claims, 8 Drawing Figures PATENTEDAPRA 21914 8801 "404 sum 2 or 4PATENIEDAPR 21914 3,801 404 sum 3 [1F 4 wsw NOVEL OPEN-CELLEDMICROPOROUS FILM CROSS-REFERENCES TO RELATED APPLICATIONS This is adivision of application Ser. No. 84,712, filed Oct. 28, 1970, now U.S.Pat. No. 3,679,538, which is a continuation-in-part of copendingapplication Ser. No. 876,511 filed Nov. 13, 1969 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to a novel process for producing open-celledmicroporous films from synthetic resins or polymers, and to the filmsproduced thereby.

Porous or cellular films can be classified into two general types: onetype in which the pores are not interconnected i. e., a closed-cellfilm, and the other type in which the pores are essentiallyinterconnected through tortuous paths which may extend from one exteriorsurface or surface region to another, i. e., an open-celled film. Theporous films of the present invention are of the latter type.

Further, the porous films of the present invention are microscopic, i.e., the details of their pore configuration or arrangement aredescernible only by microscopic examination. In fact, the open cells orpores in the films are smaller than those which can be measured using anordinary light microscope, because the wavelength of visible light,which is about 5,000 Angstroms (an Angstrom is one ten-billionth of ameter), is longer than the longest planar or surface demension of theopen cell or pore. The microporous films of the present invention may beidentified, however, by using electron microscopy techniques which arecapable of resolving details of pore structure below 5,000 Angstroms.

The microporous films of the present invention are also characterized bya reduced bulk density, sometimes hereinafter referred to simply as alow density. The bulk density is also a measure of the increase inporosity of the films. That is, these microporous films have a bulk oroverall density lower than the bulk density of corresponding filmscomposed of identical polymeric material but having no open-celled orother voidy structure. The term bulk density as used herein means theweight per unit of gross or geometric volume of the film, where grossvolume is determined by imnlstsi g a n n. mbmtth fllmin. aysssilmmxfilled with mercury at 25C. and atmospheric pressure. The volumetricrise in the level of mercury is a direct measure of the gross volume.This method is known as the mercury volumenometer method, and isdescribed in the Encyclopedia of Chemical Technology, Vol. 4, page 892(lnterscience 1949). Other important characteristics of the films of thepresent invention reside in their nitrogen flux, surface area andbreaking elongation, all of which serve to distinguish the films fromthose of the prior art.

2. Summary of the Prior Art Heretofore, films have been prepared fromsynthetic resins or polymers, e. g., polypropylene, by various meltextrusion or casting methods. Such films have many desirable propertiessuch as high strength, and resistance to heat, light, and variouschemicals.

For specific applications such as filter media and backings forbreathable medical dressings or bandages,

however, films having a porous structure in addition to their otherproperties are necessary or highly desirable.

Porous films have been produced which possess a microporous, open-celledstructure, and which are also characterized by a reduced bulk density.Films possessing this microporous structure are described, for example,in U.S. Pat. No. 3,426,754, which patent is assigned to the assignee ofthe present invention. The preferred method of preparation describedtherein involves drawing or stretching at ambient temperatures, i. e.,cold drawing, a crystalline, elastic starting film in an amount of about10 to 300 percent of its original length, with subsequent stabilizationby heat setting of the drawn film under a tension such that the film isnot free to shrink or can shrink only to a limited extent. However, thefilms of this patent are easily distinguished from those claimed hereinby the nitrogen flux and breaking elongation characteristics.

While the above described microporous or voidcontaining film of theprior art is useful, the search has continued for new processes able toproduce opencelled microporous films having a greater number of pores, amore uniform pore concentration or distribution, a larger total porearea, and better thermal stability of the porous or voidy film. Theseproperties are significant in applications such as filter media where alarge number of uniformly distributed pores are necessary or highlydesirable; and in applications such as breathable medical dressingssubject to high temperatures, e. g., sterilization temperatures, wherethermal stability is necessary or highly desirable.

SUMMARY OF THE INVENTION Accordingly, an object of the present inventionis to provide novel microporous films and processes for producing thesemicroporous polymer films which have improved porosity and thermalstability so as to prevent or substantially alleviate the limitations ordisadvantages of known porous or voidy polymer films of the prior art.

Another object of the present invention is to provide novel open-celledmicroporous polymer films having improved porosity and stability.

Other advantages and further objects of the presen invention will beapparent to those skilled in the art as the description thereofproceeds.

In accordance with the present invention, processes are provided forpreparing open-celled microporous polymer films from non-porous,crystalline, elastic polymer starting films. The process steps include(1) cold stretching, i. e., cold drawing, the elastic film until poroussurface regions or areas which are elongated normal or perpendicular tothe stretch direction are formed, (2) hot stretching, i. e., hotdrawing, the cold stretched film until fibrils and pores or open cellswhich are elongated parallel to the stretch direction are formed, andtherafter (3) heating or heat-setting the resulting porous film undertension, i.e., at substantially constant length, to impart stability tothe film.

The resulting open-celled microporous polymer films are characterized byhaving a nitrogen flux of greater than about 35.4, preferably greaterthan about 40, a bulk density lower than the density of the polymericstarting elastic material from which it is formed, usually of theelastic starting material and a surface area of at least 30 sq. m/cc.The final products formed from polypropylene also exhibit a breakingelongation of about 50 to about 150 percent.

The elastic starting film is preferably made from crystalline polymerssuch as polypropylene or oxmethylene polymers by melt extruding thepolymer into film, taking up the extrudate at a drawdown ratio giving anoriented film, and thereafter heating or annealing the oriented film ifnecessary to improve or enhance the initial crystallinity.

The essence of the present invention is the discovery that thesequential cold stretching and hot stretching steps impart to theelastic film a unique open-celled structure which results inadvantageous properties, including improved or greater porosity byreduction of bulk density, improved thermal stability and a gain orenhancement or porosity when treated with certain organic liquids suchas perchlorethylene.

As determined by various morphological techniques or tests suchaselectron microscopy, the microporous films of the present inventionare characterized by a plurality of elongated, non-porous,interconnecting surface regions or areas which have their axes ofelongation substantially parallel. Substantially alternating with anddefined by these non-porous surface regions are a plurality ofelongated, porous surface regions which contain a plurality of parallelfibrils or fibrous threads. These fibrils are connected at each of theirends to the non-porous regions, and are substantially parallel to eachother and substantially perpendicular to said axes of elongation.Between the fibrils are the pores or open cells of the films of thepresent invention. These surface pores or open cells are substantiallyinterconnected through tortuous paths or passageways which extend fromone surface region to an opposite surface area or region.

With such a defined or organized morphological structure, the films ofthe present invention have a greater proportion of surface area, agreater number of pores, and a more uniform distribution of pores, thanprevious microporous films. Further, the fibrils present in the films ofthe present invention are more drawn or oriented with respect to therest of the polymer material in the film, and thus contribute to thehigher thermal stability of the film.

Other aspects and advantages of the present invention will becomeapparent to one skilled in the art to which the present inventionpertains from the following more detailed description of preferredembodiments when read in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically illustratesapparatus suitable for carrying out the process of the presentinvention.

FIG. 2 is a photograph taken using electron microscropy techniques,which photograph is of an annealed polypropylene elastic starting filmat a magnification of 45,000.

FIGS. 3, 4, and 5 are photographs taken using electron microscopytechniques, which photographs are of a polypropylene microporous film ofthe present invention at magnifications of 13,800, 16,800 and 54,000,respectively.

FIGS. 6 and 7 are photographs taken using electron microscopytechniques, which photographs are of a polypropylene microporous filmproduced by a prior art process at magnifications of l3,800 and 5 l,000, respectively.

FIG. 8 is a graph illustrating the effect at certain heat set conditionsof various cold stretching and hot stretching operations on the rate ofnitrogen flux through microporous polypropylene films.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The microporous films of thepresent invention are formed from a starting elastic film ofcrystalline, filmforming, polymers. These elastic films have an elasticrecovery at zero recovery time (hereinafter defined) when subjected to astandard strain (extension) of percent at 25C. and 65 percent relativehumidity of at least about 40 percent, preferably at least about 50percent, and most preferably at least about 80 percent.

Elastic recovery as used herein is a measure of the ability of structureor shaped article such as a film to return to its original size afterbeing stretched, and may be calgilated as follows;

Elastic Recovery (ER) length (when stretched) -(length after stretching)ngt ad n, stir?@ 25!.v

Although a standard strain of 50 percent is used to identify the elasticproperties of the starting films, such strain is merely exemplary. Ingeneral, such starting films will have elastic recoveries higher atstrains less than 50 percent, and somewhat lower at strainssubstantially higher than 50 percent, as compared to their elasticrecovery at a 50 percent strain.

These starting elastic films will also have a percent crystallinity ofat least 20 percent, preferably at least 30 percent, and most preferablyat least 50 percent, e. g., about 50 to 90 percent, or more. Percentcrystallinity is determined by the X-ray method described by R. G. Quynnet al. in the Journal of Applied Polymer Science, Vol. 2, No. 5, pp.166-173 (1959). For a detailed discussion of crystallinity and itssignificance in polymers, see Polymers and Resins, Golding (D. VanNostrand, 1959.)

Preferred suitable starting elastic films, as well as the preparationthereof, are further defined in copending application Ser. No. 572,601,filed Aug. 15, 1966 now abandoned, Wissbrun and Bierenbaum, inventors,and assigned to the same assignee as the present invention.

Other elastic films considered suitable for the practice of the presentinvention are described in British Pat. No. 1,052,550, published Dec.21, 1966.

The starting elastic film utilized in the preparation of the microporousfilms of the present invention should be differentiated from filmsformed from classical elastomers such as the natural and syntheticrubbers. With such classical elastomers the stress-strain behavior, andparticularly the stress-temperature relationship, is governed byentropy-mechanism of deformation (rubber elasticity). The positivetemperature coefficient of the retractive force, i. e., decreasingstress with decreasing temperature and complete loss of elasticproperties at the glass transition temperatures, are particularlyconsequences of entropy-elasticity. The elasticity of the startingelastic films utilized herein, on the other hand, is of a differentnature. In qualitative thermodynamic experiments with these elasticstarting films, increasing stress with decreasing temperature (negativetemperature coefficient) may be interpreted to means that the elasticityof these materials is not governed by entropy effects but dependent uponan entry term. More significantly, the starting elastic films have beenfound to retain their stretch properties at temperatures where normalentropy-elasticity could no longer be operative.

Thus, the stretch mechanism of the starting elastic films is thought tobe based on energy-elasticity relationships, and these elastic films maythen be referred to as hon-classical elastomers.

As stated, the starting elastic films employed in this invention aremade from a polymer of a type capable of developing a significant degreeof crystallinity, as contrasted with more conventional or classicalelastic materials such as the natural and synthetic rubbers which aresubstantially amorphous in their unstretched or tensionless state.

A significant group of polymers, i. e., synthetic resinous materials, towhich this invention may be applied are the olefin polymers, e.g.,polyethylene, polypropylene, poly-3-methyl butene-l poly-4-methylpentene-l as well as copolymers of propylene, 3-me'thyl butene-l, I

4-methyl'pentene-l, or ethylene with each other or with minor amounts ofother olefins, e. g., copolymers of propylene and ethylene, copolymersof a major amount of 3-methyl butene-l and a minor amount of a straightchain n-alkene such as n-octene-l, n-hexadecene-l, noctadecene-l, orother relatively long chain alkenes, as well as copolymers of 3-methylpentene-l and any of the same n-alkenes mentioned previously inconnection with 3-methyl butene-l. These polymers in the form of filmsshould generally have a percent crystallinity of at least percent,preferably at least percent, and most preferably about 50 percent to 90percent, or higher.

For example, a film-forming homopolymer of polypropylene may beemployed. When propylene homopolymers are contemplated, it is preferredto employ an isotatic polypropylene having a percent crystallinity asindicated above, a weight average molecular weight ranging from about100,000 to 750,000 preferably about 200,000 to 500,000 and a melt index(ASTM- l958D-l238-57T, Part 9, page 38) from about 0-1 to about 75,preferably about 0.5 to 30, so as to give a final film product havingthe requisite physical properties.

While the present disclosure and examples are directed primarily to theaforesaid olefin polymers, the invention also contemplates the highmolecular weight acetal, e. g., oxymethylene, polymers. While bothacetal homopolymers and copolymers are contemplated, the preferredacetal polymer is a random oxymethylene copolymer, i. e., one whichcontains recurring oxmethylene, i.e., H2O, units interspersed with"-oii' grau s in assists par 'me'r'esarsxmaremm valent radicalcontaining at least two carbon atoms directly linked to each other andpositioned in the chain between the two valences, with any substituentson said R radical being inert, that is, which do not include interferingfunctional groups and which will not induce undesirable reactions, andwherein a major amount of the OR units exist as singleunits attached tooxymethylene groups on each side. Examples of preferred polymers includecopolymers of trioxane and cyclic ethers containing at least twoadjacent carbon atoms such as the copolymers disclosed in U.S. Pat. No.3,027,352 of Walling et al. These polymers in film form may also have acrystallinity of at least 20 percent, preferably at least 30 percent,and most preferably at least 50 percent, e. g., 50 to 60 percent, orhigher. Further, these polymers have a melting point of at least 150C.and a number average molecular weight of at least 10,000. For a moredetailed discussion of acetal and oxymethylene polymers, see,Formaldehyde, Walker, pp.l75l9l, (Reinhold, 1964).

Other relatively crystalline polymers to which the invention may beapplied are the polyalkylene sulfides such as polymethylene sulfide andpolyethylene sulfide,

' the polyarylene oxides such as polyphenylene oxide,

the polyamides such as polyhexamethylene adipamide (nylon 66) andpolycaprolactam (nylon 6), and polyesters such as polyethyleneterephthalate, all of which are well known in the art and need not bedescribed further herein for sake of brevity.

The types of apparatus suitable for forming the starting elastic filmsof this invention are well known in the art.

For example, a conventional film extruder equipped with a shallowchannel metering screw and coat hanger die, is satisfactory. Generally,the resin is introduced into a hopper of the extruder which contains ascrew and a jacket fitted with heating elements. The resin is melted andtransferred by the screw to the die from which it is extruded through aslot in the form of a film from which it is drawn by a take-up orcasting roll. More than one take-up roll in various combinations orstages may be used. The die opening or slot width may be in the range,for example, of about 10 to 200 mils.

Using this type of apparatus, film may be extruded at a drawdown ratioof about 20:1 to 200:1, preferably 50:1 to :1.

The terms drawdown ratio or, more simply, draw ratio, as used herein isthe ratio of the film wind-up or take-up speed to the speed of the filmissuing at the extrusion die.

The melt temperature for film extrusion is, in general, no higher thanabout 100C. above the melting point of the polymer and no lower thanabout 10C. above the melting point of the polymer.

For example, polypropylene may be extruded at a melt temperature ofabout C. to 270C. preferably 200C. to 240C. Polyethylene may be extrudedat a melt temperature of about 175 to 225C. while acetal polymers, e.g., those of the type disclosed in U.S. Pat. No. 3,027,352, may beextruded at a melt temperature of about C. to 235C. preferably to 215C.

The extrusion operation is preferably carried out with rapid cooling andrapid drawdown in order to obtain maximum elasticity. This may beaccomplished by having the take-up roll relatively close to theextrusion slot, e.g., within two inches and, preferably, within oneinch. An air knife operating at temperatures between, for example 0C.and 40C., may be employed within one inch of the slot to quench, i. e.,quickly cool and solidify, the film. The take-up roll may be rotated,for example, at a speed of 10 to 1000 ft/min, preferably 50 to 500ft/min.

While the above description has been directed to slit die extrusionmethods, an alternative method of forming the starting elastic filmscontemplated by this invention is the blown film extrusion methodwherein a hopper and an extruder are employed which are substantiallythe same as in the slot extruder described above.

From the extruder', the melt enters a die from which it is extrudedthrough a circular slot to form a tubular film having an initialdiameter D,. Air enters the system through an inlet into the interior ofsaid tubular film and has the effect of blowing up the diameter of thetubular film to a diameter D Means such as air rings may also beprovided for directing the air about the exterior of extruded tubularfilm so as to provide quick and effective cooling. Means such as acooling mandrel may be used to cool the interior of the tubular film.After a short distance during which the film is allowed to completelycool and harden, it is wound up on a take-up roll.

Using the blown film method, the drawdown ratio is preferably 20:1 to200:1, the slot opening 10 to 200 mils, the D /D, ratio, for example,0.5 to 6.0 and preferably about 1.0 to about 2.5, and the take-up speed,

for example, 30 to 700 ft/min. The melt temperature may be within theranges given previously for straight slot extrusion.

The extruded film may then be initially heat treated or annealed inorder to improve crystal structure, e. g., by increasing the size of thecrystallites and removing imperfections therein. Generally, thisannealing is carried out at a temperature in the range of about C. to100C. below the melting point of the polymer for a period of a fewseconds to several hours, e. g., 5 seconds to 24 hours, and preferablyfrom about 30 seconds to 2 hours. For polypropylene, the preferredannealing temperature is about 100 to 155C., while for oxymethylene(acetal) copolymers the preferred annealing temperature is about 110 to165C.

An exemplary method of carrying out the annealing is by placing theextruded film in a tensioned or tensionless state in an oven at thedesired temperature in which case the residence time is preferably inthe range of about 30 seconds to 1 hour.

The resulting partly-crystalline film is then subjected to the processof this invention to form the novel microporous films. As mentionedhereinabove, this process generally comprises the consecutive steps ofcold stretching, hot stretching and heat setting the startingnon-porous, crystalline, elastic film. This process is more specificallydescribed hereinafter.

FIG. 1 shows a schematic diagram of exemplary continuous apparatus 1suitable for production according to the present invention ofmicroporous film 2 from an elastic starting film 3. The elastic film 3from a supply or feed roll 4 is fed over an idler roll 5 into a coldstretching zone 6. The cold stretching apparatus comprises a nip roll 7cooperating with a first cold stretch roll 8'which is driven at aperipheral speed S by suitable driving means 9, and two nip rolls l0 and11 which cooperate with a second cold stretch roll 12 which is driven ata peripheral speed S which is greater than S by a suitable driving means13. The elastic film 3 is thereby cold stretched at a cold stretch ratioof 8 /8,. The cold stretched film 14 is then fed over an idler roll 5into an oven 16 which provides heat for both the hot stretching zone 17and the heat setting zone 18. The hot stretching apparatus comprises ahot stretch roll 19 driven by suitable means 13 at a peripheral speed Swhich is about the same or slightly, e. g., less percent, greater than Sto prevent relaxation of the cold stretched film 14. The hot stretchroll 19 cooperates with nip roll 21 so as to provide sufficientfrictional engagement. Idler rolls 22 may be provided to achieve desiredresidence time in the oven and yet minimize necessary oven capacity. Asecond hot stretch roll 23 is driven by driving means 20 at a peripheralspeed 8,, which is greater than S The cold stretched film 15 is therebyhot stretched at a hot stretch ratio of 8 /8 The cold stretched-hotstretched film 24 is passed around idler rolls 25 to achieve sufficientresidence time for heat setting, and is then passed about a take-up roll26 and a nip roll 27 and collected on a conventional takeup roll 28. Thetake-up roll 26 is driven by driving means 20 at about the same speed ashot stretch roll 23 as to maintain the film in tension during heatsetting.

The term cold stretching as used herein is defined as stretching ordrawing a film to greater than its original length and at a stretchingtemperature, i. e., the temperature of the film being stretched, lessthan the temperature at which melting begins when the film is uniformlyheated from a temperature of 25C. and at a rategf 20C. per r ninute.Theterm hot stretching V as used herein is defined as stretching abovethe temperature at which melting begins when the film is unifonnlyheated from a temperature of 25C. and at a rate of 20C. per minute, butbelow the normal melting point of the polymer, i.e., below thetemperature at which fusion occurs. As is known to those skilled in theart, the temperature at which melting begins and the fusion temperaturemay be determined by a standard differential thermal analyzer (DTA), orby other known apparatus which can detect thermal transitions of apolymer.

The temperature at which melting begins varies with the type of polymer,the molecular weight distribution of the polymer, and the crystallinemorophology of the film. For example, polypropylene elastic film may becold stretched at a temperature below about 120C. preferably betweenabout 10C. and C. and conveniently at ambient temperature, e. g., 25C.The cold stretched polypropylene film may then be hot stretched at atemperature above about C. and below the fusion temperature, andpreferably between about C. and about C. Again, the temperature of thefilm itself being stretched is referred to herein as the stretchtemperature. The stretching in these two steps or stages must beconsecutive, in the same direction, and in that order, i. e., cold thenhot, but may be done in a continuous, semi-continuous, or batch process,as long as the cold stretched film is not allowed to shrink to anysignificant degree, e.g., less than 5 percent of its cold stretchedlength, before being hot stretched.

The sum total amount of stretching in the above two steps may be in therange of about 10 to 300 percent and preferably about 50 to 150 percent,based on the initial length of the elastic film. Further, the ratio ofthe amount of hot stretching to the sum total amount of stretching ordrawing may be from above about 0.10:1 to below 0.99:1, preferably fromabout 0.50:1 to 0.97:1, and most preferably from about 0.60:1 to 0.95:1.This relationship between the cold" and hot stretching is referred toherein as the extension ratio (percent hot extension to the percenttotal" extension).

in any stretching operations where heat must be supplied the film may beheated by heat supplied by the moving rolls which may in turn be heatedby an electrical resistance method, by passage over a heated plate,through a heated liquid, a heated gas, or the like.

After the above-described two stage or two step stretching, thestretched film is heat set. This heat treatment may be carried out at atemperature in the range from about 125C. up to less than the fusiontemperature, and preferably about 130 to 160C. for polypropylene; fromabout 80C. up to less than fusion temperature, and preferably about 140to 160C., for acetal polymers; from about 75C. up to less than fusiontemperature, and preferably about 1 15 to 130C., for polyethylene, andat similar temperature ranges for other of the above mentioned polymers.This heat treatment should be carried out while the film is being heldunder tension, i. e., such that the film is not free to shrink or canshrink to only a controlled extent not greater than about 15 percent ofits stretched length, but not so great a tension as to stretch the filmmore than an additional 15 percent. Preferably, the tension is such thatsubstantially no shrinkage or stretchingoccurs, e. g., less than percentchange in stretched length.

The period of heat treatment which is preferably carried outsequentially with and after the drawing operation, shouldnt be longerthan 0.1 second at the higher annealing temperatures and, in general,may be within the range of about 5 seconds to 1 hour and preferablyabout 1 to 30 minutes.

The above described setting steps may take place in air, or in otheratmospheres such as nitrogen, helium or argon.

FIG. 2 is a photograph taken using electron microscopy techniques, i.e., a micrograph or photomicrograph, of a non-porous annealedpolypropylene elastic starting film at a magnification of 45,000. Thisfilm is a portion of a starting film produced as described incomparative EXAMPLES I-IX herein. As can be seen from FIG. 2, this filmshows no distinguishing structural features or characteristics, i. e.,the surface of the film is relatively smooth and homogeneous.

On the other hand, distinguishing structural features are clearly shownin the microporous films shown in the micrographs of FIGS. 3 to 7.

Moreover, the distinguishing structural features of a microporous filmof the present invention, shown in FIGS, 3 to 5 at magnifications of13,800, 16,800 and respectively, are clearly different from thedistinguishing structural features of a microporous film, shown in FIGS.6 and 7 at magnifications of 13,800

. and 51,000, respectively, produced by a process as described in theabove-mentioned US. '-Pat. No. 3,426,754.

Referring to FIGS. 3 to 5, the microporous film shown therein is aportion of a microporous film of the present invention produced asdescribed'in EXAMPLE VII herein. As can be seen from FIGS. 3 to 5, themicroporous films of the present invention have a plurality ofelongated, non-porous,interconnecting surface regions of areas A whichhave their axes of elongation substantially parallel to each other, andsubstantially normal or perpendicular to the direction in which the filmis stretched or drawn accordingto the'process of the present invention.Substantially alternating with and defined by the non-porous surfaceregions A is a plurality of elongated, porous surface regions B whichcontain a plurality of parallel fibrils C. The fibrils C areconnected ateach of their ends to the non-porous regions A, and are substantiallyperpendicular to them.

Between the fibrils C are the pores D, which appear white or whitish, ofthe film.

one-half of that of the microporous film of the present invention shownin FIGS. 3 to 5.

The above discussed micrographs were taken using the electron microscopytechnique described in Geils Polymer Single Crystals, page 69Interscience 1963), and are considered as true reproductions.

The microporous films of the present invention, in a tensionless state,have a lowered bulk density compared with the density of correspondingpolymeric elastic materials having no-open-celled structure, e. g. thosefrom which it is formed. Thus the films have a bulk density nogreaterthan percent and preferably 50 to 75 percent of the elasticstarting material. Stated another way, the bulk density has been reducedby at least 5 percent and preferably 15 to 50 percent. Forpolypropylene, the reduction is 15 to 42 percent, preferably 38 percent,and for polyethylene, 34 to 41 percent. The bulk density is also ameasure of porosity that is, where the bulk density is about 50 to 75percent of the starting material, the porosity has been increased by 50to 25 percent because of the pores or holes.

The final crystallinity of the microporous film is preferably at least30 percent, more preferably at least 40 percent, and more suitably about50 to percent, as determined by the aforementioned X-ray method.

The microporous films of the present invention may also have an averagepore size of 100 to 5,000 Angstroms, and more usually to 3,000Angstroms, the values being determined by mercury porosimetry, asdescribed in an or by R. G. Quynn, on pages 21-34 of Textile ResearchJournal, January, 1963.

Ein'ilifiyj'th microporous filni of this invention has,

at 25C. and 65 percent relative humidity, an elastic recovery from a 50percent extension of 60 to 85 percent, a tensile strength of 20,000 to30,000 psi, a modulus of 100,000 to 300,000 psi (all the foregoing inthe machine direction), and a haze of 30 percent to opaque, depending onfilm thickness. The elastic recovery is an important characteristic asit distinguishes the films from prior art films of the same type inwhich openings have been placed therein by processes other than theinvention. Films formed from polypropylene also have a breakingelongation of 50 to 150 percent.

The valuesof recovery of elastic recovery hereinbefore referred to areelastic values determined as extended. A reading was recorded as soon asa no-load.

condition was indicated by the testing machine. The elastic recovery isthen calculated as follows:

Elastic Recovery 5 The other properties iiien tiBriEd were 'deterniiiied with a standard ASTM method as follows:

Tensile Strength ASTM No. D882 Method A (Sample width 15 mm) BreakingElongation ASTM No. D882 Method A (Sample width 15 mm) Modulus ASTM No.D882 Method A (Sample width 1 inch) Haze ASTM No. D1003 Procedure A asper FIG.

2. Further, polypropylene microporous film of the present inventionexhibits water vapor transmittance as high as 1800, generally 500 to1400, the units of transmittance being given in cc/24 hours-m -atm, themethod of determining transmittance being ASTME 96-63T (Procedure B).

A further important distinguishing characteristic of the microporousfilms of the present invention with respect to those of the prior artresides in the nitrogen flux determination.

The values of nitrogen flux referred to are calculated as follows:

A film having a standard surface area of 6.5 cm is mounted in a standardmembrane cell having a standard volume of 63 cm, and the cell ispressurized to a standard differential pressure (the pressure dropacross the film) of 200 psi with nitrogen. The supply of nitrogen isthen closed off and the time required for the pressure to drop to afinal differential pressure of 150 psi as the nitrogen permeates throughthe film is measured with a stop watch. The nitrogen flux, Q,(in gmol/cmminXl is then determined from the equation:

AT=elapsed time (in seconds) T=temperature of nitrogen (in K) which isderived from the gas law, PV=ZnRT.

The novel microporous films of the present invention advantageouslyexhibit a nitrogen flux or Q value of at least 35.4, more preferably avalue of at least 40 and highly preferably a value of from about 50 to300. With respect to specific microporous films, the nitrogen flux forfilms formed from polypropylene or polyethylene ranges from about 50 to200 and highly preferably about 100 in optimum products.

As can be seen from TABLE 1 following, optimum nitrogen flux and thermalstability for polypropylene are obtained when the extension ratio isgreater than 0.60:1 and less than 1.0:1. Further, the advantages of thepresent invention can be illustrated by comparison of identical elasticfilm stretched 100 percent hot (extension ratio of 1.0:1) or 100 percentcold (extension ratio of 0.1) with one where as little as 5 percent coldstretch (extension ratio of0.95:1) prior to the hot stretch givesexcellent porosity and stability.

When novel microporous films of the present invention are prepared whichexhibit the required nitrogen flux and porosity values, that is anitrogen flux of at least 35.4 and preferably 50 to 300, and bulkdensity about 50 to 75 percent of the bulk density of the correspondingpolymer film having no open-celled structure,

the resulting films will also be found to have a surface area withincertain predictable limits. This surface area value or characteristic isinherent in the films when theythey also have the nitrogen fl ux a ndFdu ced bulk density values given above. Thus, in the films of thepresent invention when nitrogen flux values and bulk density values areas indicated, they will also be found to have a surface area of at least30 sq.m/cc. and preferably in the range of about 40 to 200 sq.m/cc. Forfilms formed from polypropylene, the surface area generally ranges fromabout 30 to l 10 sq.m/cc. and preferably about 60 sq.m/cc. Formicroporous polyethylene films prepared according to the presentinvention, the

surface area range is about 30 to 35 sq.m/cc.

I certain preferred embodiments of the invention but it is not to beconsidered as limited thereto.

EXAMPLES i-Ix Crystalline polypropylene having a melt index of 0.7 and adensity of 0.92 was melt extruded at 230C. through an 8 inch slit die ofthe coat hanger type using a 1 inch extruder with a shallow meteringscrew. The length to diameter ratio of the extruder barrel was 24/1. Theextrudate was drawndown very rapidly to a melt drawdown ratio of 150,and contacted with a rotating casting roll maintained at 50C. and 0.75inches from the lip of the die. The film produced in this fashion wasfound to have the following properties: thickness, 0.001 inches;recovery from 50 percent elongation at 25 C., 50.3 percent;crystallinity, 59.6 percent.

A sample of this film was oven annealed with air with a slight tensionat C. for about 30 minutes, removed from the oven and allowed to cool.It was then found to have the following properties: recovery from a 50percent elongation at 25C., 90.5 percent; crystallinity, 68.8 percent.An electron micrograph of a portion of this intermediate or elasticstarting film is shown in FIG. 1.

Samples of the annealed elastic film were then subjected to variousextension ratios as shown in TABLE I, and thereafter heat set undertension, i.e., at constant length, at C. for 10 minutes in air. The colddrawing portion was conducted at 25C., the hot drawing portion wasconducted at 145C., total draw was 100 percent, based on the originallength of the elastic film. Thermal stability was determined bymeasuring nitrogen flux after various intervals of residence at 65C. Theresults are summarized in Table 1.

TABLE I Nitrogcn Flux (g mole/cm min X 109) of Mieroporous PolypropyleneFilm after 65C. Storage Extension Initial Flux Flux Flux Flux Flux FluxFlux After After After After After After After EXAMPLE Ratio Flux 1.0hr. 30 hr. 48 hrs. 87 hrs. 159 hrs. 281 hrs. 252 hrs.

1 a) 0.0 35.4 ll 0.-l0 45.5 0 Ill 0.20 46.7 0 1V 0.40 61.7 1.33 0

V 0.60 76.1 34.8 21.5 1.54 0 VI 0.80 100 79.9 71.0 42.5 36.8 33.0 27.724.5 Vll 0.90 127.5 106 100 71.0 66.0 61.7 56.3 split Vlll 0.95 113 10087 7 I .0 61.7 59.8 54.7 56.3 lX h) 1.0 19.7 13.7 11.9 7.8 5.5 split a)100% cold stretch h) 100% hot stretch EXAMPLES X TO XVlll Examples l-lXwere repeated except the heat-set conditions of temperature and timewere varied, using 130C. for 5 minutes, and 150C. for minutes. Theireffect on the rate of nitrogen flux is shown graphically in FIG. 8,along with the results of EXAMPLES I-lX.

As can be seenfrom FIG. 8, higher flux values were obtained usingconsecutive cold and hot stretching steps at each of the heat-setconditions than either type of stretching used alone.

EXAMPLE XIX To characterize the uniqueness of the microporous filmobtained by the present invention, 4 inch by 3% inch samples of coldstretched microporous films of 1 mil caliper thickness, were produced bycold (at C.) stretching a portion of the elastic film described in EX-AMPLE l to 100 percent of its original length and then heat setting thecold stretched film under tension, i. e., at its stretched length, for10 minutes at about 140C.

As can be seen from TABLE 11, treatment of a microporous polypropylenefilm prepared by the cold 0 stretch/hot stretch process withperchlorethylene caused a dramatic increase, e.g., over 45 percent, inthe porosity of the film as measured by the increase in nitrogen flux(hereinafter referred to as the P. R. Value). There was a loss innitrogen flux, however, when the cold stretch process film and the hotstretch process film were treated with the same solvent, as evidenced bynegative P. R. Values of 2.38 percent and 68.9 percent, respectively.

In general, the microporous films of the present invention have zero orpositive, i. e., greater than zero, P. R. Values and these P. R. Valuesmay, for example, range from 0 to 100 percent, and more usually from 20to percent.

EXAMPLE XX The film-forming polymer of this example is a copolymer oftrioxane and 2 weight percent, based on the weight of the polymer, ofethylene oxide of the type described in U.S. Pat. No. 3,027,352, whichis aftertreated to remove unstable groups as described in U.S. Pat. No.3,219,623, and which has a melt index of 2.5.

The above-described polymer is melt extruded at 195C. through an 8 inchslit of the coat hanger type using a 1 inch extruder with a shallowchannel metering screw. The length to diameter ratio of the extruderbarrel was 24:1. The extrudate is drawn down to a drawdown ratio of150:1, contacted with a rotating casting roll maintained at about 145C.,and about one-quarter inch from the lip of the die. The film produced inthis manner is wound up and found to have the following properties: thiekness 0.000 5 inc hes reeovery from 5 0 TABLE I1 EFFECT OF SOLVENT ONNITROGEN FLUX* ON MICROPOROUS POLYPROPYLENE FILMS Cold Streteh/ ColdStretch Process 100% Hot Stretch Proceitx Hot Stretch Process BeforeAfter Before T" Aft?" Before After Treatment Treatment P.R. TreatmentTreatment P.R. Treatment Treatment P.R. Solvent X10(F,) X10 01 Value"X10" X10" Value l0-"(F IO"(F Value Pcrchloro ethylene 46.65 45.54 "2.38%39.85 12.40 68.9% I74 319 +455 g-mols/cmlmin P. R.'Value(pcrchloroethylene reaction value) F F IF X 100% percent strain, 45percent. The polymer is then oven annealed in the tensionless state at145C. for 16 hours. At the end of the annealing period it is removedfrom the oven, allowed to cool and found to have the followingproperties: thickness, 0.0005 inches; recovery from 50 percentelongation, 92 percent.

The film is cold stretched at 25C. to 10 percent of EXAMPLE XXICrystalline polyethylene having a density of 0.96 and a melt index of0.7 is melt extruded at 195C. through V a 4 inch diameter annular diehaving an opening of 0.04 inches. The hot tube thus formed is expanded1.5 times by internal air pressure and cooled by an air stream impingingon the film from an air ring located around and above the die. Theextrusion is accomplished with an extruder of 24:1 length to diameterratio and a shallow channel metering screw. The extrudate is drawn downto a drawdown ratio of 100:1 and passed through a series of rollerswhich collapses the tube. After wind-up the film is oven annealed in atensionless state at 1 15C. for 16 hours.

After removal from the oven, the film is allowed to cool, and stretchedat an extension ratio of 0.80, by 50 percent of its original length withcold stretching being conducted at 25C. and hot stretching beingconducted at 1 15C., and heat set in the oven at constant length for 5minutes at 120C, after which it is found to have the open-celledmicroporous structure of the present invention.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein, however, is notto be construed as limited to the particular forms disclosed, sincethese are to be regarded as illustrative rather than restrictive.Variations and changes may be made by those skilled in the art withoutdeparting from the spirit of the present invention.

What is claimed is:

l. A process for the preparation of an open-celled microporous polymerfilm characterized by having a reduced bulk density as compared to thebulk density of the corresponding polymer films having no open celledstructure, a crystallinity of above about 30 percent, a pore size ofless than 5000 Angstroms, a nitrogen flux of greater than about 35.4, asurface area of at least 30 sq.m/cc. and a breaking elongation of 50 to150 percent, which process comprises:

cold stretching a non-porous, crystalline, elastic film until poroussurface regions perpendicular to the stretch direction are formed, thenon-porous elastic film having crystallinity of above about 20 percent,and an elastic recovery from a 50 percent strain of at least 40 percentat 25C.,

hot stretching the resulting cold-stretched film until 7 pore spaceselongate parallel to the stretch direction are formed,

and thereafter heating the resulting microporous film under tension. 2.A process according to claim 1 wherein the polymer film has a bulkdensity of about 50 to percent of the bulk density of correspondingpolymer films having no open-celled structure, a nitrogen flux of about40 to 300 and a surface area of about 40 to 200 sq.m/cc.

3. The process of claim 2 wherein the cold stretching and the hotstretching are conducted at an extension ratio between about 0.10:1 and0.99:1, and wherein the polymer is selected from the group consisting ofpolyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfidesand polyarylene oxides.

4. A process for the preparation of an open-celled microporouspolypropylene film characterized by hav ing reduced bulk density ascompared to the bulk density of the corresponding polymer films havingno open-celled structure, a crystallinity of above about 30 percent, apore size of less than 5000 Angstroms a nitrogen flux of greater than35.4 and a breaking elongation of 50 to 150 percent, which comprises:

cold stretching at a temperature between 10C. and

70C. a non-porous, crystalline, elastic, polypropylene film to developporous surface regions perpendicular to the stretch direction, theelastic polypropylene film having an initial crystallinity of at least30 percent and an initial elastic recovery from a 50 percent strain ofat least 50 percent at 25C., hot stretching the resulting unrelaxcd coldstretched film at a temperature between C. and C. to a total stretch ofl() to 300 percent of the original length of the elastic film and at anextension ratio between 0.50:1 and 0.97:1 to develop pore spaceselongated parallel to the stretch direction, the cold stretching and thehot stretching being conducted in the same stretch direction, and

thereafter heating the resulting microporous film at substantiallyconstant length at a temperature between about 130C. and C.

5. The process of claim 4 wherein the polypropylene has a molecularweight of from 200,000 to 500,000 and a melt index of from about 0.5 to30, and wherein the film is subjected to a total stretch of 50 to 150percent of its original length and at an extension ratio between 0.60:1and 0.95:1.

6. A process according to claim 5 wherein the resulting polypropylenefilm has a bulk density of about 58 to 70 percent of the correspondingpolypropylene film have no open-celled structure, a nitrogen flux ofabout a 50 to 200 and a surface area of about 30 to 110 sq.m/ce.

2. A process according to claim 1 wherein the polymer film has a bulkdensity of about 50 to 75 percent of the bulk density of correspondingpolymer films having no open-celled structure, a nitrogen flux of about40 to 300 and a surface area of about 40 to 200 sq.m/cc.
 3. The processof claim 2 wherein the cold stretching and the hot stretching areconducted at an extension ratio between about 0.10:1 and 0.99:1, andwherein the polymer is selected from the group consisting ofpolyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfidesand polyarylene oxides.
 4. A process for the preparation of anopen-celled microporous polypropylene film characterized by havingreduced bulk density as compared to the bulk density of thecorresponding polymer films having no open-celled structure, acrystallinity of above about 30 percent, a pore size of less than 5000Angstroms a nitrogen flux of greater than 35.4 and a breaking elongationof 50 to 150 percent, which comprises: cold stretching at a temperaturebetween 10*C. and 70*C. a non-porous, crystalline, elastic,polypropylene film to develop porous surface regions perpendicular tothe stretch direction, the elastic polypropylene film having an initialcrystallinity of at least 30 percent and an initial elastic recoveryfrom a 50 percent strain of at least 50 percent at 25*C., hot stretchingthe resulting unrelaxed cold stretched film at a temperature between130*C. and 150*C. to a total stretch of 10 to 300 percent of theoriginal length of the elastic film and at an extension ratio between0.50:1 and 0.97:1 to develop pore spaces elongated parallel to thestretch direction, the cold stretching and the hot stretching beingconducted in the same stretch direction, and thereafter heating theresulting microporous film at substantially constant length at atemperature between about 130*C. and 160*C.
 5. The process of claim 4wherein the polypropylene has a molecular weight of from 200,000 to500,000 and a melt index of from about 0.5 to 30, and wherein the filmis subjected to a total stretch of 50 to 150 percent of its originallength and at an extension ratio between 0.60:1 and 0.95:1.
 6. A processaccording to claim 5 wherein the resulting polypropylene film has a bulkdensity of about 58 to 70 percent of the corresponding polypropylenefilm have no open-celled structure, a nitrogen flux of about a 50 to 200and a surface area of about 30 to 110 sq.m/cc.